The present invention relates to a solid electrolytic capacitor and, more particularly, to a solid electrolytic capacitor using a conductive polymer compound as a solid electrolyte.
A tantalum solid electrolytic capacitor is generally constituted by an anode obtained by sintering a tantalum powder, a tantalum oxide coating serving as a dielectric formed on the anode, a solid electrolyte formed on the oxide coating, and a cathode conductor formed on the solid electrolyte.
A conventional capacitor using manganese dioxide as a solid electrolyte is well known. In recent years, for example, as disclosed in Japanese Patent Publication No. 4-56445 (Reference 1), a capacitor using a conductive polymer compound such as polypyrrole in place of manganese dioxide is also proposed. Note that, as cathode conductors, a graphite layer and a silver layer are often used.
A capacitor using a conductive polymer compound as a solid electrolyte has excellent characteristics in a high-frequency range compared with that of a capacitor using manganese dioxide because the conductivity of the conductive polymer compound is several tens times that of manganese dioxide. A capacitor using the conductive polymer compound has received a good deal of attention as a capacitor which copes with an increase in operating frequency of electronic equipment in recent years. Since manganese dioxide is produced by thermally decomposing manganese nitrate, thermal stress at about 200.degree. to 300.degree. C. repeatedly acts on a capacitor element, and defects occur in an oxide coating, thereby disadvantageously increasing a leakage current. In contrast to this, when a conductive polymer compound is used as a solid electrolyte, it is known that a highly reliable product which is free from degradation of an oxide coating can be provided because a capacitor element need not be treated at a high temperature (e.g., Reference 1). As types of conductive polymer compounds, polypyrrole, a derivative thereof (Reference 1 and Japanese Patent Laid-Open No. 64-4912 (Reference 2)), and a five-membered heterocyclic copolymer or a mixture of polypyrrole, polythiophene, and a derivative thereof as disclosed in, e.g., Japanese Patent Laid-Open No. 2-98915 (Reference 3 ) are used.
Although a solid electrolytic capacitor using a conductive polymer compound has excellent high-frequency characteristics compared with a tantalum solid electrolytic capacitor using only manganese dioxide as a solid electrolyte, the solid electrolytic capacitor using a conductive polymer compound poses several problems because the solid electrolyte consists of a single chemical species. In particular, when the conductive polymer compound is to be used as the solid electrolyte of the electrolytic capacitor, a balance between a conductivity and a heat resistance is important. More specifically, although both polypyrrole and polyaniline are attempted to be applied as the solid electrolytes of solid electrolytic capacitors, polypyrrole generally has a high conductivity and a low heat resistance, but polyaniline generally has a low conductivity and a high, heat resistance. The conductivity directly influences high-frequency characteristics, and the heat resistance influences the application temperature of a capacitor.
In order to solve the above problems, it is proposed to use, as a solid electrolyte, a copolymer or a mixture (for example, a copolymer or mixture of pyrrole, N-methylpyrrole, N-ethylpyrrole, and the like (Reference 1)), having the same basic skeleton. However, the effect of this proposal is not satisfactory. On the other hand, as described above, it is also proposed to use, as a solid electrolyte, as a five-membered heterocyclic copolymer or a mixture of polypyrrole, polythiophene, and derivatives thereof. However, since these constituent elements have no significant difference in heat resistance and conductivity, as described in Reference 3, only the high-frequency characteristics of the capacitor using the above copolymer or mixture as a solid electrolyte are improved compared with those of a capacitor using manganese dioxide as a solid electrolyte.